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Power-factor and insulation-resistance measurements are aids in controlling the quality of transformer insulation in the factory. These measurements are affected by a number of factors, which must be considered in their evaluation. Some of these factors are related to the quality of the insulation and some are not. Moisture is a harmful contaminant that may be detected by these measurements. An experimental investigation of the effect of moisture on the power factor, insulation resistance, dielectric absorption characteristics, and the dielectric strength is reported. A correlation has been found between moisture content, as indicated by these measurements, and the 1-minute 60-cycle strength, the 7-hour 60-cycle strength, and the impulse strength of oil impregnated press-board. The results of a number of power-factor measurements on completed oil, Inerteen, and air-insulated transformers are summarized. Finally, there is a description of how dielectric measurements are applied in helping to control the quality of transformer insulation in the factory.